NASA Sets Spacewalk to Repair Discovery's Heat Shield

HOUSTON -
The crew of the space shuttle Discovery will perform an unprecedented on-orbit
repair Wednesday, sending an astronaut under the orbiter's belly to remove a two strips of material jutting out from its tile-covered
heat shield, mission managers said Monday.

The
decision caps three days of scrutiny by imaging specialists, shuttle tile
engineers, aerodynamicists and spacewalk planners to determine exactly how to
deal with the two bits of ceramic fiber cloth - known as gap-fillers
- sticking out from between heat-resistant tiles under the forward section of
the Discovery orbiter.

"In the end
it came down to be a really simple decision," said Wayne Hale, NASA's deputy shuttle
program manager, during a briefing here at NASA's Johnson Space Center (JSC).
"We came to the conclusion that we don't know enough to really feel good about
this, so therefore the remedy is easy and we ought to go exercise the remedy."

That remedy
is an unprecedented spacewalk
to send one astronaut, attached to a robotic arm, under the shuttle to pluck
out the strips by hand, or cut them off if they prove too stubborn, Hale said.
The extravehicular activity (EVA), currently expected to take about 90 minutes,
will be folded into the timeline for the third and final spacewalk of
Discovery's STS-114 mission, an early morning extravehicular activity set for
Aug. 3, he added.

"I think
it's a fairly simple task," Cindy Begley, lead EVA officer for Discovery's
flight, said of the repair plan earlier today. "It's just making sure we're not
going to hit the vehicle, and we're doing that."

Hale said
that while the gap-filler repair is slated for an Aug. 3 spacewalk, an extra
day added to the STS-114 mission on Sunday allows room to push the EVA back
a day should the shuttle crew and spacewalk planners request it.

"We would
take the extra transfer day that we've added...and move it in front of the EVA
instead of in back of it if they request," Hale said.

The two
gap-fillers jutting from Discovery's belly were not caused by impacts or foam
debris from the shuttle's external tank, and likely shook loose from the
thin glue mount that connects them to the orbiter's undercarriage, shuttle
officials said. The ceramic material is used to keep the orbiter's
heat-resistant tiles from clattering against one another during launch, as well
as to fill in excess space between individual tiles, they added.

If the
filler material sticks out from between tiles during reentry, they can disrupt
the aerodynamic flow around the orbiter during reentry, causing higher than
normal local heating on the order of hundreds of degrees Fahrenheit, said Chuck
Campbell, a NASA subsystem engineer who studies the heating issues associated
with shuttle reentry.

After a
thorough study, aerodynamic heating specialists were unable to say that the two
protruding gap-fillers seen on Discovery would not pose a danger to its crew during
the orbiter's planned Aug. 8 reentry through the Earth's atmosphere.

"Given that
large degree of uncertainty, life could be normal during reentry or some bad
things could happen," Hale said. "To set our minds at rest...the EVA team has
been off working hard for the last three days."

During a
morning status report, Begley said the EVA team planned
to use the robotic arm aboard the International Space Station (ISS) to maneuver
STS-114 astronaut Stephen Robinson underneath Discovery with his tools and
tethers strapped close behind him to ensure he did not accidentally strike the
vital heat-resistant tiles. Using his own, spacesuit-clad he fingers, he would
then pluck the gap-fillers one at a time, she added.

Robinson
will also carry a saw tool, scissors and forceps should the gap-fillers prove
too tough to yank out by hand, Begley has said.

The gap-filler
removal will likely take place after Robinson and his spacewalking partner
Soichi Noguchi, of the Japanese Aerospace Exploration Agency (JAXA), install
the External Stowage Platform 2 - a spare parts fixture for the ISS - to the
station's Quest module, shuttle officials said.

Since the
repair is a solo task, Noguchi will be free to retrieve a faulty rotary motor
from the ISS and perform other tasks while Robinson removes and gap-filler. The
two astronauts have already tested out potential heat tile and reinforced
carbon carbon panel repairs in their first
spacewalk on Saturday. They also retrieved the tools Robinson will use in
the gap-filler removal during an early morning spacewalk
today.

Meanwhile,
shuttle engineers are already looking at how to redesign gap-fillers and their
installation processes to prevent such protrusions in future orbiter flights,
NASA officials said.

Shuttle
astronauts have repaired their own orbiters via spacewalks three times in the
past, once to replace a broken television camera and twice to properly stow a
Ku-band antenna, Hale said.

"We're
committed to safety," Hale said, adding that Discovery's flight is paving new
ground for NASA's shuttle program. "We're building up a new database and we're
going to be smarter and safer in the future because we have this new
knowledge."

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